ModelRailroadForums.com is a free Model Railroad Discussion Forum and photo gallery. We cover all scales and sizes of model railroads. Online since 2002, it's one of the oldest and largest model railroad forums on the web. Whether you're a master model railroader or just getting started, you'll find something of interest here.
I'm not sure what that would be in scale terms though. If your modeling HO then it would be some where around 1 11/16" (real measurement but I think 1 3/4" would work) between bents, give or take. Therefore, you would need a "small support bent either end and 2 along the length of the bridge.
I followed the directions on my first trestle I built and I’m not near it to measure. I’ve built a few trestles now and I believe I settled for what looked right or maybe I looked in my bridges and trestles book.
Ok just looking in my layout topic I grabbed a pic of a trestle , I know this trestle is 3 feet long in HO scale so if you divide up the number of bents it’ll give you an idea, I count 13 bents . I can look better and measure if you like when I get home.
I pulled out MRs Bridges and Trestles by Jeff Wilson and could not find the span spacing. Strange? Maybe I just missed it. The link provided by NWP Dave has a drawing indicating 12 feet.
After looking at a great many pictures of trestles I have come to the conclusion that there is no standard distance between bents. As such, you could get away with a spacing of anything between 10' to 15'. Basically, whatever looks best on your layout for the trestle.